The invention concerns a scanning image which comprises at least two kinds of scanning elements having different properties and which is produced in a thermotransfer process. It further concerns a thermotransfer foil for the production of a scanning image of that kind, in which the transfer layer which can be transferred from a carrier film onto the substrate, to produce the different scanning elements, has a number, corresponding to the number of different scanning elements, of regions which are respectively associated with a kind of scanning element and which are of correspondingly different natures.
The known thermotransfer printing processes, for producing half-tone images, usually operate with a scanning procedure, wherein scanning elements or points of normally equal size are transferred from the thermotransfer foil onto the substrate, in a scanning element density which differs according to the desired brightness of the scanning image. If multi-colour scanning images are to be produced, this procedure involves using thermotransfer foils whose transfer layer is respectively subdivided into a plurality of regions, wherein associated with each colour is a specific region of the, transfer layer. In the printing operation the thermotransfer foil is then moved over the substrate, in a manner corresponding to the desired colour, and coloured scanning elements are produced by means of the printing tool, wherein generally the differently coloured regions of the transfer layer of the thermotransfer foil correspond in terms of their dimensions, to the substrate to be printed upon.
In that way it is possible to produce scanning or raster images of good quality, when using a correspondingly close scanning raster and small scanning elements. Nonetheless, in the known procedure, either the possible design configurations are limited, or operation must be conducted with very small scanning elements or points and very small scanning element spacings, so that the apparatus expenditure becomes very high. The production of partly matt and partly shiny or reflecting scanning images has not been considered hitherto.
The object of the present invention therefore is to develop further possible design configurations for scanning images, without having to involve a particularly high level of apparatus expenditure.
To attain that object, in accordance with the invention it is proposed that a scanning image of the kind set forth in the opening part of this specification is such that at least two kinds of scanning elements are of respectively different dimensions. When the scanning image is of such a configuration, to produce half-tones it is no longer necessary for the spacing of the scanning elements or the density thereof to be altered. If there is the possibility of providing scanning elements of different dimensions, which is a possibility which has hitherto never yet been used, then regions of the scanning image can be produced with a lower level of colour density by virtue of using scanning elements of smaller diameter while, when a full or deep colour or a good covering effect is to be achieved, scanning elements of larger diameter are used. That variation in the scanning element size is advantageous in particular when the scanning elements are of a specific structure and for example are reflecting as in such a situation the variation in the scanning element size provides for a particularly uniform effect in regard to the respective structure involved.
Further possible configurations for the scanning image are afforded if at least two kinds of scanning elements or points are each of a different, optically effective structure. For example a scanning image can be composed of elements or points with a matt surface and elements or points with a shiny surface, whereby that permits not only the usual half-tone or colour resolution of a scanning image, but it also affords the possibility of constructing the scanning image by different shine effects etc. That gives quite specific scanning images which differ from the previously known scanning images and which are particularly difficult to imitate and which cannot be reproduced for example by means of a colour copier, which means that such scanning images are particularly suitable for example as security elements for value-bearing documents such as for example banknotes, credit cards, identity cards or passes or the like which in fact are increasingly the subject of attempts at forgery, in particular using modern colour copiers.
It is particularly advantageous if the optically effective structure of at least one kind of scanning elements is a diffraction structure which produces diffraction or interference, preferably a grating structure. The most widely varying optical effect can be generated by means of diffraction or interference structures of that kind, the respective structure to be used depending on whether the scanning image is observed in a reflecting light mode or in a transmission light mode.
By means of different structures, and this is known per se, it is for example also possible to form a scanning image in the form of an optically variable image, more specifically in such a way that the scanning image changes in dependence on the lighting or viewing angle or the wavelength of the light used for lighting purposes, in which case only the colour position varies in the simplest form. In such a situation, using two kinds of scanning elements of different diffraction structures, by means of which for example alphanumeric characters are produced, can provide that the colour of the characters on the one hand and the background on the other hand alter in dependence on the viewing angle or the light used for illumination purposes.
In order to enhance diffraction or interference effects of that kind, it is desirable for at least one kind of scanning elements to be provided with a reflecting layer whereby those elements are of a corresponding level of brightness. By using a reflecting layer in relation to only one kind of scanning elements, it can further be provided that those scanning elements appear substantially brighter relative to the other scanning elements forming the scanning image, whereby it is possible to achieve graphic effects which were hitherto unknown in relation to scanning images. It will be appreciated however that it is also possible for all scanning elements forming the scanning image to be of a reflecting character, but for them each to be provided with a respectively different structure, for example for given kinds of the scanning elements to be formed with a grating structure while other scanning elements have a flat reflecting layer.
Finally it will be appreciated that it is also possible for at least two kinds of scanning elements to be of respectively different colours, whereby the possible configurations are additionally increased.
A thermotransfer foil of the kind set forth in the opening part of this specification for the production of a scanning image according to the invention is distinguished in that the transfer layer in the different regions has scanning elements of different dimensions in order for example always to be able to work with the same scanning element density, while however nonetheless having the possibility of producing locations of the substrate image on the substrate, which locations involve denser or less dense printing.
A thermotransfer foil can also desirably be such that the different regions of the transfer layer each involve a respective optically differently effective structure. To produce the scanning image the respective scanning elements are then transferred onto the substrate from the different regions of the transfer layer with the structure that has different optical effects, for which purpose the thermotransfer foil must be moved relative to the substrate in the manner known from thermo-colour printers, in order to bring the respective region of the transfer layer which has the desired surface structure into a position over the corresponding location of the substrate.
Particular effects can be achieved if the transfer layer has a reflecting layer at least in one region, wherein the reflecting layer is desirably formed by a metallisation because then the scanning image can be composed of reflecting and non-reflecting regions or, if all regions of the transfer layer are of a reflecting nature, it is possible to produce images of particular brightness.
That is of significance in particular if the optically effective structure of the transfer layer is a diffraction structure for producing diffraction or interference, in particular a grating structure.
In order to produce scanning images of suitable durability, it may be desirable if, in at least one region, adjoining the carrier film, the transfer layer has a transparent protective lacquer layer, because that can then increase the abrasion resistance of the scanning image which is produced on the substrate.
When there is a transparent protective lacquer layer, that layer can advantageously have colours which are different in at least two regions of the transfer layer, thereby affording the possibility of producing multi-colour scanning images.
The optically effective structure of the transfer layer is advantageously produced by it being impressed or stamped into a lacquer layer of the transfer layer. Corresponding stamping processes are known from the production of hot stamping foils with diffraction structures etc. In that case the structures are impressed or stamped by means of a die into a thermoplastic lacquer or a lacquer which has not completely hardened. That process can in principle be applied in the same manner to thermotransfer foils or the transfer layers thereof, in which case it can be at most necessary to adapt the structure depth to the area of use, because the thickness of the transfer layer of thermotransfer foils is limited, in order to guarantee satisfactory transfer of the transfer layer onto the substrate, using the known apparatuses.
Finally it may be advantageous if the protective lacquer layer covers the optically effective structure when the transfer layer is applied to a substrate because that then makes it difficult if not even impossible to take a casting therefrom and thus produce a forgery. At the same time that increases the durability of the scanning image because the surface structure is protected from direct mechanical attacks.
In regard to the basic structure of the transfer layer of the thermotransfer foil, attention can be directed to per se known foils as well as hot stamping foils, in which respect the point to be emphasised as the only difference in the thermotransfer foil according to the invention in comparison with known thermotransfer foils is that, in the case of the thermotransfer foil according to the invention, structuring of the surface of the transfer layer which is to be transferred onto the substrate must be effected at least in one region, and for that reason a suitably deformable layer must be provided. Further details relating to the composition of the layers and the thicknesses thereof are set out hereinafter.